Some advanced helicopter rotor systems have flaps on the rotor blades that act as secondary control surfaces and affect the aerodynamic characteristics of the blades. These are similar in nature to the flaps provided in fixed wing aircraft wings although they are generally smaller and more compact due to the smaller size of the rotor blades. These flaps can be used for improving the efficiency of the rotor blades by modifying the lift and by reducing the vibrations on the rotor.
Each flap requires an actuator to control its movement. Movement may be controlled by the pilot either directly or via an electronic controller. In the case of anti-vibration system the latter is more likely, with rotor flaps being automatically controlled based on the outputs of various sensors.
In certain circumstances it may be desirable to disable these flaps. This may be an intentional flight decision not to use them or it may be desirable to disable them in case of system failure.
Flaps will generally be provided on each of several rotor blades and there may be more than one flap per blade. It may be desirable to disable all or only some of the flaps.